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I have always wondered the differences between a wing made to hold 2 engines and a wing made to hold 4 engines, could someone please explain?

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    $\begingroup$ Hum... what do you want to know? It is about why the engine number varies from one aircraft to the other... and possibly whether wings need to be adjusted according to the number of engines; or really about possible basic difference in the wing, regardless of the reason of 2 or 4 (or even more) engines? $\endgroup$ – mins Jan 13 '17 at 16:44
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    $\begingroup$ I don't have an answer for this, but some relevant information may be available around the wings for A330 and A340. They differ mostly be the number of engines, and the differences in the wings will be large due to this difference. $\endgroup$ – Monolo Jan 13 '17 at 16:48
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    $\begingroup$ @Monolo -- actually, the A330 and A340 use the same wing structure -- in the A330's case, its simply missing the outboard pylons and engines. (This came in handy when the A330 MRTT was developed as the outboard hardpoints were reused for refueling pods) $\endgroup$ – UnrecognizedFallingObject Jan 13 '17 at 23:22
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    $\begingroup$ @mins Or fewer -- most wings hold zero, one or two engines. ;-) :-P $\endgroup$ – David Richerby Jan 16 '17 at 10:11
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The differences are surprisingly small, and the implications of two versus four engines are not very great as far as design of the wing is concerned.

Weight implications

The wing is an immensely strong structure - after all, it's able to bear the weight of the entire aircraft, including of course the engines.

Engines are heavy, but only a small fraction of the weight of the whole aircraft.

A similar aircraft with four engines rather than two will have smaller engines, so the total weight difference will not be large.

Space implications

You need more space to fit two engines on each side, but not a vast amount. For example, the British Aerospace 146 has four remarkably small Lycoming ALF 502 engines. What's more, four small engines can be easier to mount, because they need less ground clearance.

Same wings, more engines

In practice, it's not very difficult to add engines to a wing (for aircraft companies, that is; I don't think I'd be able to do it without seeing someone do it first at least once).

For example, the 747 can optionally carry a fifth engine - obviously, this engine is just being carried, not used, but it shows that simply placing the engine there doesn't present insurmountable aerodynamic or other engineering difficulties.

And then we have the Airbus A330/A340, designed and built in parallel, which are essentially the same aircraft - same fuselage and same wing - one with two engines and one with four. (Actually the -500 and -600 A340 variants do have slightly longer wings, but that's because they are heavier, and nothing to do with the additional engines.)

That's not to say that there are no implications in having additional engines. In this case, wing flutter caused by the outboard engines of the A340 required some stiffening.

And of course, additional engines will require additional fuel and control lines. However, these aren't part of the structure of the wing itself, and are not in themselves likely to have implications for it.

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  • $\begingroup$ I think the weight implications might be larger to you suggest, engines on the wings can cause quite some bending moment relief (thereby lightening the wing structure) $\endgroup$ – ROIMaison Jan 16 '17 at 9:49
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    $\begingroup$ The A340-500/600 also has a different sweep angle and also a twist that the A340-200/300 and A330 doesn't have, but otherwise you are correct. The only real implication an additional engine has on the wing is actually the ability to carry more fuel in the outer wing tanks, because the bending moment is damped by the extra engine... $\endgroup$ – Moo Jan 23 '17 at 0:40
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Wings on commercial airplanes are complex, they have electrical lines, hydraulics, fuel tanks, lines and pumps plus structure to handle the forces involved in flight. A wing that has engines on it will need additional structure for the engine to mount on, and have to be stronger in some ways because of the weight of the engine and the thrust it produces. So a wing that has engines on it will need to be stronger than one without, and therefore will be heavier and possibly thicker at points.

The difference between 2 engines and 4 is probably less than having no engines and 2 engines, but there are considerations. The weight of the outer engine will have a bigger moment arm than the inner engine, so the wing will need to be stronger to compensate. The additional engine controls and fuel cross-feed lines will need more space in the wing than a wing with 1 engine, perhaps making the wing thicker or bigger than it would be on one with 1 engine.

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    $\begingroup$ In many respects a wing with an engine on it is stressed less than if the same engine was mounted elsewhere. In flight, an engine on the wing is right where lift is generated, so the force to carry it does not need to be transmitted all the way to the fuselage (as would be the case for a body-mounted engine). On the ground, a wing-mounted engine must be carried by the wing, of course, but this is still less load than the fact that when airborne the wing carries the entire fuselage! (Though in the latter case the bending moment goes in the opposite direction). $\endgroup$ – Henning Makholm Jan 13 '17 at 19:57
  • $\begingroup$ @HenningMakholm you're correct that the engine mass is quite helpful in the wing root bending moment. The joints and surrounding structure is much affected by the thrust and crash landing forces of the engines. But still a distributed load on the wing is always a good thing, which must be a trade off against the added complexity, number of serviced elements, cost, economies etc. $\endgroup$ – Gürkan Çetin Jan 14 '17 at 8:15

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